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anyone w/similar problems on MUA?

Got a greenheck m#VSU MAU that fails to light intermitantly. Have done everything suggested by manufactures, no help. Now suggest to convert to 120v ignition at xxxx dollars just for parts. Existing fenwal ignition system,amplifier,maxitrol mod valve,& honeywell main valve. Have verified gas press.,press. drop across burn,adjust low fire bypass on maxitrol. Guess my question is is there something I might be missing that somone else might catch or suggest to resolve problem without spending xxxx $ in just parts and end up with same problem afterward?

You don't have another similar unit onsite that you could start swaping ignition components with, and see if the problem follows do you?? You probably would have already thought of this, just throwing it out there!

Got a greenheck m#VSU MAU that fails to light intermitantly. Have done everything suggested by manufactures, no help. Now suggest to convert to 120v ignition at xxxx dollars just for parts. Existing fenwal ignition system,amplifier,maxitrol mod valve,& honeywell main valve. Have verified gas press.,press. drop across burn,adjust low fire bypass on maxitrol. Guess my question is is there something I might be missing that somone else might catch or suggest to resolve problem without spending xxxx $ in just parts and end up with same problem afterward?

What options you got? Is is 100% fresh air? 80/20? Does it have a burner bypass damper? Is it setup for variable airflow?

You have a 24v ignition system? All the ones I have ever worked on where 120v spark ignition with a RM7800 primary with flame rectification. What exact ignition is in there?

thanks for replies. I have already swaped out the fenwall ignition control,ignitor,cleaned burner,ohmed out ignitor wire, & thought I had it. Following day got word was locked out. It is 100% FA. Has VFD and is there for easy start. When it fails, it fails on ignition start. Did replace flame sensor also. Have followed manufactures' installation instructions to the T, and they themselves have been helpfull, but would like to cover every base before I convince owner into spending xxxx $ and still have same problem afterward!

thanks for replies. I have already swaped out the fenwall ignition control,ignitor,cleaned burner,ohmed out ignitor wire, & thought I had it. Following day got word was locked out. It is 100% FA. Has VFD and is there for easy start. When it fails, it fails on ignition start. Did replace flame sensor also. Have followed manufactures' installation instructions to the T, and they themselves have been helpfull, but would like to cover every base before I convince owner into spending xxxx $ and still have same problem afterward!

So what was your velocity across the burner and what size unit do you have?

drop across burner .75" . 206,200 max in & 16,000 min I believe it's a VSU-110-H20. guess to answer I did not check actual velocity. believe velocity would be right if drop across burner is in close paramaters of manufacture which is 0.65, or am I wrong.

Are there any other safeties that reset that deenergize the gas and pilot valve?
A pair of cambridge units I work on will have a limit switch that will open and lock out the heating relay that allows the pilot and main gas valves to operate. This causes the fenwal 24vac hsi ignition control to lock out on flame fail when it tries to relight.

the only thing is since direct fired it only has single try and if unsuccessful it is locked out on flame fail & then goes out on freeze protect. It does have hi-temp limit, but it modulates and runs fine. Just not when it sometimes fails to light.

I inherrited an account with a fire tube boiler, that for years went out on flame failure sporadically, I went through every possible situation, after days of studying the situation I began noticing on extremely cold mornings it would trip, on mild mornings it would not. I began checking inlet gas pressures on various temped mornings and finally caught the inlet pressure supplied by the city on extremely cold mornings dropping low enough to cause flame failure, of course after speaking with the city, it became a gas demand issue, they ended up replacing the city block regulator and increased the supply pressure and in turn "NO MORE FLAME FAILURES" As a temporary fix I increased my gas regulator pressure to max allowable inlet pressure which was high enough on most cold mornings to keep flame failure from occuring. Sometimes looking at the condition the system is in such as manufacturing etc... find out when its tripping what other loads are sharing gas line etc.... Outside the box so to speak.